Water heating apparatus

ABSTRACT

A water heating apparatus is provided. The water heating apparatus includes a pump and is configured to be able to set a hot-water circulation route to circulate hot water discharged from a heat exchanger to be re-entered into the heat exchanger by operation of the pump. When a state of a hot-water supply operation in which a burner drives combustion is changed to an end state, hot water in the heat exchanger is circulated in the hot-water circulation route to suppress deposition of scale.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Japan application serial no. 2020-146681, filed on Sep. 1, 2020. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND Technical Field

The disclosure relates to a water heating apparatus such as a hot-water supply apparatus, and more particularly, to a type of water heating apparatus using a burner and a heat exchanger.

Description of Related Art

Patent Document 1 (Japanese Patent Application Laid-Open No. 2014-88876) discloses a specific example of a water heating apparatus. The water heating apparatus described in Patent Document 1 includes a burner and a heat exchanger which performs heat recovery from a combustion gas generated by the burner, and water supplied to the heat exchanger is heated to generate hot water. In such a water heating apparatus, the burner is driven and burned to perform a hot-water supply operation, and when the hot-water supply operation is ended, there is a phenomenon in which the hot water in the heat exchanger is heated to a high temperature by the residual heat of the heat exchanger. Here, when the hot water is heated to a high temperature, scale is likely to deposit from the hot water. With low thermal conductivity, scale not only deteriorates the performance of the heat exchanger, but may also cause clogging in the heat exchanger. Therefore, it is desired to take measures to prevent scale from depositing as much as possible. As a means for this purpose, Patent Document 1 adopts a means of sending unheated water into the heat exchanger to cool the heat exchanger when the hot-water supply operation in which the burner is driven and burned is ended. According to such a means, heating of the hot water in the heat exchanger to a high temperature is suppressed, and deposition of scale is suppressed.

However, in the above related art, there is still room for improvement as described below.

That is, in the above related art, the water sent into the heat exchanger to cool the heat exchanger is drained by using a drain pipe, i.e., being disposed of. This causes a lot of waste of water resources and is not preferable for natural environment protection through resource conservation.

SUMMARY

The following technical means are adopted in the disclosure.

An embodiment of the disclosure provides a water heating apparatus including a burner, a heat exchanger, and a hot water flow path. The heat exchanger performs heat recovery from a combustion gas generated by driving combustion of the burner. The water flow path is connected to the heat exchanger in a manner enabling entry of water into the heat exchanger and discharge of heated hot water. The water heating apparatus includes a pump and is configured to be able to set a hot-water circulation route to circulate hot water discharged from the heat exchanger to be re-entered into the heat exchanger by operation of the pump. When a state of a hot-water supply operation in which the burner drives combustion is changed to an end state, hot water in the heat exchanger is circulated in the hot-water circulation route to suppress deposition of scale.

According to such a configuration, the following effects can be obtained. That is, when a state of a hot-water supply operation in which the burner drives combustion is changed to an end state, since the hot water in the heat exchanger circulates in the determined hot-water circulation route, different from the case where hot water continues to stay in the heat exchanger, it is possible to suppress heating of the hot water in the heat exchanger to a high temperature. As a result, it is difficult for scale to deposit, and issues such as a decrease in thermal efficiency and clogging due to the deposition of scale can be solved. Also, in the disclosure, water is circulated, and different from Patent Document 1, water for cooling the heat exchanger is not discarded. Therefore, it contributes to eliminating waste of water resources and protecting the natural environment by conserving resources.

In an embodiment of the disclosure, the water heating apparatus may include a control means storing a data which defines a pump operation start condition for operating the pump during execution of the hot-water supply operation. When the pump operation start condition is satisfied during execution of the hot-water supply operation, operation of the pump may be started by control of the control means.

According to such a configuration, since the operation of the pump may be started before the hot-water supply operation is ended, compared to the case where the operation of the pump is started after the hot-water supply operation is ended, the replacement of the hot water present in the heat exchanger can be quickly performed after the hot-water supply operation is ended. As a result, it is possible to more thoroughly prevent heating of the hot water in the heat exchanger to a high temperature by the residual heat of the heat exchanger. By setting an appropriate condition signaling in advance the end of the hot-water supply operation as the pump operation start condition, it is possible to prevent the pump from being operated for an unnecessarily long time during execution of the hot-water supply operation.

In an embodiment of the disclosure, during execution of the hot-water supply operation, when an incoming-water flow rate of incoming water to the heat exchanger decreases from a state exceeding a predetermined flow rate to the predetermined flow rate, operation of the pump may be started, where the predetermined flow rate is a value larger than a minimum operating flow rate of the burner.

According to such a configuration, the following effects can be obtained. That is, for example, when hot water is supplied from the water heating apparatus to a faucet which is a hot-water supply terminal, the hot-water supply operation ends as the faucet is operated, its opening degree gradually decreases, and the flow rate of incoming water to the heat exchanger also gradually decreases. According to the above configuration, the operation of the pump can be appropriately started before the hot-water supply operation is ended as the flow rate of incoming water to the heat exchanger decreases. If the pump is operated before the end of the hot-water supply operation, as described above, it is possible to more reliably suppress heating of the hot water in the heat exchanger to a high temperature after the end of the hot-water supply operation.

In an embodiment of the disclosure, during execution of the hot-water supply operation, when an incoming-water flow rate of incoming water to the heat exchanger decreases and a decrease range or a decrease rate is equal to or greater than a predetermined value, operation of the pump may be started.

As an advanced signal of the end of the hot-water supply operation, it is conceivable that the amount of water used by the user is significantly reduced, or the amount of water used is rapidly reduced. According to the above configuration, when such a situation occurs, the operation of the pump is started. Therefore, it is possible to increase the possibility that the operation of the pump will be started immediately before or near the end of the hot-water supply operation.

In an embodiment of the disclosure, an operating speed of the pump may be higher after the hot-water supply operation is ended than during execution of the hot-water supply operation.

According to such a configuration, since the operating speed of the pump is low during execution of the hot-water supply operation, it is possible to reduce waste. After the hot-water supply operation is ended, the pump will be operated at a high speed, so the flow rate of hot water circulating in the hot-water circulation route increases, i.e., the flow rate of water flowing through the heat exchanger increases, and it is suitable for preventing heating of the hot water in the heat exchanger to a high temperature.

In an embodiment of the disclosure, in a case where the hot-water supply operation continues without being ended although a predetermined first time has elapsed since operation of the pump is started, operation of the pump may be stopped.

According to such a configuration, it is possible to adequately prevent the pump from wastefully operating for a long period of time.

In an embodiment of the disclosure, the water heating apparatus may include a fan capable of supplying air to the burner. The fan may operate even after driving combustion of the burner is stopped to cause air to act on the heat exchanger.

According to such a configuration, it is possible to obtain the effect of cooling the heat exchanger by using the air sent from the fan, which contributes to further preventing heating of the hot water in the heat exchanger to a high temperature.

In an embodiment of the disclosure, after operation of the pump is started, at the time when a temperature of hot water flowing out of the heat exchanger drops to a predetermined temperature or below, or at the time when a predetermined second time has elapsed since the hot-water supply operation is ended, operation of the pump may be stopped.

According to such a configuration, it is possible to adequately prevent the pump from operating for an unnecessarily long period of time.

In an embodiment of the disclosure, the water heating apparatus may include a flow rate sensor provided at a position in the hot water flow path avoiding the hot-water circulation route to serve as a detection means of an incoming-water flow rate of incoming water to the heat exchanger. The flow rate sensor may be used for determination of a minimum operating flow rate of the burner.

According to such a configuration, after the hot-water supply operation is ended, even if the hot water in the heat exchanger circulates in the hot-water circulation route by operation of the pump, it is possible to prevent detection of the flow of the hot water in the hot-water circulation route by the flow rate sensor. Therefore, even if the incoming-water flow rate of incoming water to the heat exchanger is equal to or higher than the minimum operating flow rate of the burner, there is no concern that the burner would drive combustion on this basis. This contributes to reducing the total number of flow rate sensors and simplifying the driving combustion control of the burner.

Other features and advantages of the disclosure will become more apparent from the following description of embodiments of the disclosure with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of a water heating apparatus according to the disclosure.

FIG. 2 is a schematic main part view showing an example of an operating state of FIG. 1.

FIG. 3 is a flowchart showing an example of an operation processing procedure in the water heating apparatus of FIG. 1.

DESCRIPTION OF THE EMBODIMENTS

According to an embodiment of the disclosure, it provides a water heating apparatus capable of appropriately preventing, without causing waste of water, heating of hot water in a heat exchanger to a high temperature and deposition of a large amount of scale when a hot-water supply operation is ended.

Hereinafter, exemplary embodiments of the disclosure will be specifically described with reference to the drawings.

A water heating apparatus WH shown in FIG. 1 is configured as a hot-water supply apparatus capable of performing general hot-water supply to a faucet 9. Specifically, the water heating apparatus WH includes a fan 1, a burner 2, a heat exchanger 3, a hot water flow path 4, a control part 5, and various sensors and valves to be described later.

The burner 2 is, for example, a gas burner involving a total primary combustion method or the like. The fan 1 supplies the burner 2 with a mixture of a fuel gas and air, or air for burning a fuel gas. In this embodiment, a reverse combustion method in which the combustion gas travels downward is shown, but the disclosure is not limited thereto. The heat exchanger 3 has a configuration in which a heat transfer tube 30 made of metal such as stainless steel or copper is housed in a case 31 to heat the water flowing in the heat transfer tube 30 by performing heat recovery from the combustion gas using the heat transfer tube 30. Reference numeral 90 in FIG. 1 indicates an exhaust duct for discharging to the outside the combustion gas for which heat recovery has been completed.

The hot water flow path 4 includes an incoming-water flow path 40, a hot-water discharge flow path 41, a bypass flow path 42, and an external hot-water discharge flow path 43, and as shown in FIG. 2, a hot-water circulation route R using a pump P may be set. Specifically, the incoming-water flow path 40 is a flow path for guiding unheated water, which is supplied from a water supply pipe such as a running water pipe to a water inlet 40 a, to an inlet part of the heat exchanger 3. The hot-water discharge flow path 41 is a flow path which guides the hot water discharged from an outlet part of the heat exchanger 3 to a hot-water outlet 41 a and may supply the hot water to the external hot-water discharge flow path 43 provided with a hot-water supply terminal such as the faucet 9. The bypass flow path 42 connects halfway positions of the incoming-water flow path 40 and the hot-water discharge flow path 41 and includes a flow rate adjustment valve V1. In a normal hot-water supply operation, the bypass flow path 42 mixes a part of the unheated water flowing through the incoming-water flow path 40 with the heated hot water in the hot-water discharge flow path 41, and by adjusting the mixing ratio, the hot-water discharge temperature at the hot-water outlet 41 a can be controlled to a desired target hot-water supply temperature.

As shown in FIG. 2, the hot-water circulation route R is configured using parts of the incoming-water flow path 40 and the hot-water discharge flow path 41, the pump P provided at the hot-water discharge flow path 41, the bypass flow path 42, and the heat exchanger 3. In the hot-water circulation route R, by the operation of the pump P, hot water circulates in a fixed route as shown by arrows N1 to N4 in FIG. 2. In that case, by flowing out of the heat exchanger 3 and flowing through the hot-water circulation route R, the hot water in the heat exchanger 3 re-enters the heat exchanger 3 and is circulated.

The control part 5 is configured by using, for example, a microcomputer or the like, and corresponds to an example of a control means in the disclosure. As will be described later, the control part 5 executes operation control and data processing of each part of the water heating apparatus WH. The operation control target also includes the pump P, and a data D of a condition (pump operation start condition) for starting the operation of the pump P in a hot-water supply operation is stored in advance in a storage part 50 of the control part 5. A specific example of the pump operation start condition will be described later.

In addition to another flow rate control valve V2, the hot water flow path 4 is provided with a flow rate sensor Sa for detecting an incoming-water flow rate, a temperature sensor Sb for detecting a temperature of incoming-water to the heat exchanger 3, and a temperature sensor Sc for detecting a temperature of hot water discharged from the heat exchanger 3, and various detection signals are transmitted from the above devices to the control part 5. Here, the flow rate sensor Sa is used for the determination of a minimum operating flow rate (MOQ) of the burner 2. The “minimum operating flow rate” refers to a minimum water flow rate at which the burner 2 starts to drive combustion. Therefore, if the water flow rate detected by the flow rate sensor Sa is less than the minimum operating flow rate, the burner 2 does not drive combustion. Also, the flow rate sensor Sa is provided at a position avoiding the hot-water circulation route R shown in FIG. 2. Therefore, as shown in FIG. 2, when hot water circulates in the hot-water circulation route R, the flow rate sensor Sa does not detect the flow rate and the burner 2 does not drive combustion.

Next, an example of an operation processing procedure in the water heating apparatus WH will be described with reference to the flowchart of FIG. 3, and the effects thereof will also be described.

First, when the faucet 9 is opened, a flow of water equal to or more than the minimum operating flow rate (MOQ, Q means flow rate) is generated in the hot water flow path 4, and an MOQ-on state is detected through the flow rate sensor Sa, the burner 2 drives combustion and an hot-water supply operation is started (S1: YES, S2). At this time, the fan 1 is also driven.

Next, when an incoming-water flow rate Q of the incoming water to the heat exchanger 3 changes and decreases to a predetermined flow rate Qa from a situation in which a large amount of water exceeding the predetermined flow rate Qa is flowing, the control part 5 starts the operation (low speed operation) of the pump P at this time (S3: YES, S4). The above predetermined flow rate Qa is a value larger than the minimum operating flow rate. According to the above control, in the process in which the user gradually closes the faucet 9 from a wide-open state and the incoming-water flow rate Q decreases, the operation of the pump P is started before the driving combustion of the burner 2 is stopped. The condition of step S3 corresponds to a specific example of the “pump operation start condition” in the disclosure. This is also true for the condition of step S11 to be described later.

According to the control as described above, when the operation of the pump P is started, the hot-water circulation route R as shown in FIG. 2 is set. The hot-water supply operation is ended by closing the faucet 9, and as a result, it becomes an MOQ-off state in which the incoming-water flow rate Q is less than the minimum operating flow rate (MOQ), and the driving combustion of the burner 2 is stopped. After the hot-water supply operation is ended, upon setting of the hot-water circulation route R, since replacement of the hot water in the heat exchanger 3 is performed, heating of the hot water in the heat exchanger 3 to a high temperature by the residual heat of the heat exchanger 3 is appropriately suppressed. When the hot water is heated to a high temperature, scale is deposited from the hot water. The deposition amount of scale increases as the temperature of the hot water increases. In this regard, according to this embodiment, it is possible to suppress heating of the hot water to a high temperature and suppress deposition of scale. Since the operation start time of the pump P is before the end of the hot-water supply operation, at the time when the hot-water supply operation is ended, the pump P may be in a steady operation state, and it is possible to prevent occurrence of a start-up delay of the pump P.

When the hot-water supply operation is ended, the operating speed of the pump P is made higher than the speed up to that time point (S5: YES, S6). As a result, it is possible to increase the flow rate of the hot water in the hot-water circulation route R and effectively perform replacement of the hot water in the heat exchanger 3. In contrast, during the hot-water supply operation, since the pump P is operated at a low speed, in addition to keeping the running cost of the pump P low, it is also possible to prevent an excessive amount of hot water supplied from the pump P to the faucet 9. In addition, even after the hot-water supply operation is ended, the fan 1 is not stopped and its operation is continued. This makes it possible to cool the heat exchanger 3 by blowing air from the fan 1 to further suppress an increase in the hot-water temperature in the heat exchanger 3 to a high temperature and promote a drop in the hot-water temperature in the heat exchanger 3 at an early stage.

Different from the above, even if the incoming-water flow rate Q of the incoming water to the heat exchanger 3 does not decrease to the above predetermined flow rate Qa, when the incoming-water flow rate Q decreases and the decrease range or the decrease rate is equal to or greater than a predetermined value, the operation of the pump P is started (S3: NO, S11: YES, S4). When the user closes the faucet 9 and stops the hot-water supply, in many cases, the opening degree of the faucet 9 is significantly reduced or the faucet 9 is closed rapidly in advance, and even in these cases, the operation of the pump P is started before the hot-water supply operation is ended. Therefore, similar to the above case, when the hot-water supply operation is ended thereafter, heating of the hot water in the heat exchanger 3 to a high temperature and deposition of a large amount of scale are suppressed.

After the above operation control is executed, when the temperature of the hot water discharged from the outlet part of the heat exchanger 3 drops to a predetermined temperature or below, the operations of the pump P and the fan 1 are stopped (S7: YES, S8). According to such control, the pump P and the fan 1 may be suitably operated, and the situation that “ the hot-water temperature in the heat exchanger 3 has not dropped sufficiently while the pump P and the fan 1 stopped” won't happen. The pump P and the fan 1 are also prevented from being operated for an unnecessarily long time.

On the other hand, even if the hot-water discharge temperature does not drop to the predetermined temperature or below, when a predetermined second time elapses from the end of the hot-water supply operation, the operations of the pump P and the fan 1 are stopped at this time (S7: NO, 512: YES, S8). According to such control, it is possible to surely prevent the pump P and the fan 1 from being operated for an unnecessarily long time.

Different from the above operation, after the operation of the pump P is started in step S4, it is likely that the hot-water supply operation does not end at an early stage and a predetermined first time elapses, and in that case, the operation of the pump P is stopped (S5: NO, S9: YES, S10). According to such control, it is possible to prevent the pump P from being wastefully operated for a long time during the hot-water supply operation.

As described above, according to the water heating apparatus WH, after the hot-water supply operation is ended, it is possible to appropriately suppress an increase in the hot-water temperature in the heat exchanger 3 to a high temperature and deposition of a large amount of scale. Also, as a means for preventing an increase in the hot-water temperature in the heat exchanger 3 to a high temperature, the hot water in the heat exchanger 3 is circulated in the hot-water circulation route R. Therefore, different from the case where water is sent to the heat exchanger 3 for cooling and the water used for this cooling is discarded, for example, the disclosure may eliminate waste of water resources and protect the natural environment by conserving resources.

The disclosure is not limited to the contents of the above embodiment. The specific configuration of each part of the water heating apparatus according to the disclosure may be modified in design in various manners within the scope of the disclosure.

In the disclosure, in place of the flow rate sensor Sa, for example, as shown by a virtual line in FIG. 2, a flow rate sensor Sa′ located at the hot-water circulation route R may be provided. However, according to such a configuration, when hot water circulates in the hot-water circulation route R, since the flow rate sensor Sa′ performs flow rate detection, the flow rate sensor Sa′ cannot be used for determination of the minimum operating flow rate (MOQ) of the burner 2, and it will be necessary to separately use another flow rate sensor.

In the disclosure, the operation of the pump may also be started at the same time as the end of the hot-water supply operation. However, since it generally takes a certain amount of time for the pump to start up, in the case of the above configuration, the replacement operation of the hot water in the heat exchanger tends to be delayed. Therefore, compared to the case where the operation of the pump is started before the end of the hot-water supply operation, the temperature of the hot water in the heat exchanger may be slightly higher, but it is possible to obtain the effect intended in the disclosure.

In the above embodiment, the contents of steps S3 and S11 correspond to specific examples of the pump operation start condition for operating the pump during execution of the hot-water supply operation, but the disclosure is not limited thereto. What condition is set as the pump operation start condition is a matter that may be appropriately determined by, for example, the manufacturer of the water heating apparatus. The hot-water circulation direction of the hot-water circulation route R may be set in a direction opposite to the directions of arrows N1 to N4 in FIG. 2. During the hot-water supply operation, since unheated water flows to the bypass flow path 42, for example, when one end side (the left end side of FIG. 1 and FIG. 2) of the bypass flow path 42 is provided in an arrangement close to the outlet part of the heat exchanger 3, by setting the circulation direction of the water in a direction opposite to the directions of arrows N1 to N4, the unheated water in the bypass flow path 42 may be quickly sent into the heat exchanger 3.

The type of the burner is not particularly limited, and for example, an oil burner may also be used. The specific type and configuration of the heat exchanger are also not particularly limited. The water heating apparatus according to the disclosure may also be configured as a water heating apparatus capable of bath hot-water supply, heating and hot-water supply, etc., in addition to serving as a hot-water supply apparatus capable of performing general hot-water supply. 

What is claimed is:
 1. A water heating apparatus comprising: a burner; a heat exchanger for performing heat recovery from a combustion gas generated by driving combustion of the burner; a hot water flow path connected to the heat exchanger in a manner enabling entry of water into the heat exchanger and discharge of heated hot water; and a pump, wherein a hot-water circulation route is settable, the hot-water circulation route circulates hot water discharged from the heat exchanger to be re-entered into the heat exchanger by operation of the pump, and when a state of a hot-water supply operation in which the burner drives combustion is changed to an end state, hot water in the heat exchanger is circulated in the hot-water circulation route to suppress deposition of scale.
 2. The water heating apparatus according to claim 1, comprising a control means storing a data which defines a pump operation start condition for operating the pump during execution of the hot-water supply operation, wherein when the pump operation start condition is satisfied during execution of the hot-water supply operation, operation of the pump is started by control of the control means.
 3. The water heating apparatus according to claim 1, wherein during execution of the hot-water supply operation, when an incoming-water flow rate of incoming water to the heat exchanger decreases from a state exceeding a predetermined flow rate to the predetermined flow rate, operation of the pump is started, wherein the predetermined flow rate is a value larger than a minimum operating flow rate of the burner.
 4. The water heating apparatus according to claim 2, wherein during execution of the hot-water supply operation, when an incoming-water flow rate of incoming water to the heat exchanger decreases from a state exceeding a predetermined flow rate to the predetermined flow rate, operation of the pump is started, wherein the predetermined flow rate is a value larger than a minimum operating flow rate of the burner.
 5. The water heating apparatus according to claim 1, wherein during execution of the hot-water supply operation, when an incoming-water flow rate of incoming water to the heat exchanger decreases and a decrease range or a decrease rate is equal to or greater than a predetermined value, operation of the pump is started.
 6. The water heating apparatus according to claim 2, wherein during execution of the hot-water supply operation, when an incoming-water flow rate of incoming water to the heat exchanger decreases and a decrease range or a decrease rate is equal to or greater than a predetermined value, operation of the pump is started.
 7. The water heating apparatus according to claim 3, wherein during execution of the hot-water supply operation, when an incoming-water flow rate of incoming water to the heat exchanger decreases and a decrease range or a decrease rate is equal to or greater than a predetermined value, operation of the pump is started.
 8. The water heating apparatus according to claim 2, wherein an operating speed of the pump is higher after the hot-water supply operation is ended than during execution of the hot-water supply operation.
 9. The water heating apparatus according to claim 3, wherein an operating speed of the pump is higher after the hot-water supply operation is ended than during execution of the hot-water supply operation.
 10. The water heating apparatus according to claim 5, wherein an operating speed of the pump is higher after the hot-water supply operation is ended than during execution of the hot-water supply operation.
 11. The water heating apparatus according to claim 2, wherein in a case where the hot-water supply operation continues without being ended although a predetermined first time has elapsed since operation of the pump is started, operation of the pump is stopped.
 12. The water heating apparatus according to claim 3, wherein in a case where the hot-water supply operation continues without being ended although a predetermined first time has elapsed since operation of the pump is started, operation of the pump is stopped.
 13. The water heating apparatus according to claim 5, wherein in a case where the hot-water supply operation continues without being ended although a predetermined first time has elapsed since operation of the pump is started, operation of the pump is stopped.
 14. The water heating apparatus according to claim 8, wherein in a case where the hot-water supply operation continues without being ended although a predetermined first time has elapsed since operation of the pump is started, operation of the pump is stopped.
 15. The water heating apparatus according to claim 1, comprising a fan capable of supplying air to the burner, wherein the fan operates even after driving combustion of the burner is stopped to cause air to act on the heat exchanger.
 16. The water heating apparatus according to claim 2, comprising a fan capable of supplying air to the burner, wherein the fan operates even after driving combustion of the burner is stopped to cause air to act on the heat exchanger.
 17. The water heating apparatus according to claim 1, wherein after operation of the pump is started, at the time when a temperature of hot water flowing out of the heat exchanger drops to a predetermined temperature or below, or at the time when a predetermined second time has elapsed since the hot-water supply operation is ended, operation of the pump is stopped.
 18. The water heating apparatus according to claim 2, wherein after operation of the pump is started, at the time when a temperature of hot water flowing out of the heat exchanger drops to a predetermined temperature or below, or at the time when a predetermined second time has elapsed since the hot-water supply operation is ended, operation of the pump is stopped.
 19. The water heating apparatus according to claim 1, comprising a flow rate sensor provided at a position in the hot water flow path avoiding the hot-water circulation route to serve as a detection means of an incoming-water flow rate of incoming water to the heat exchanger, wherein the flow rate sensor is used for determination of a minimum operating flow rate of the burner.
 20. The water heating apparatus according to claim 2, comprising a flow rate sensor provided at a position in the hot water flow path avoiding the hot-water circulation route to serve as a detection means of an incoming-water flow rate of incoming water to the heat exchanger, wherein the flow rate sensor is used for determination of a minimum operating flow rate of the burner. 